1. Field of the Invention
This invention relates generally to antennas and more particularly to a novel lightweight relatively low frequency linear element antenna of relatively wide bandwidth.
2. Prior Art
As is well known to those versed in the antenna art, the ability of an electrical conductor to serve as a radiator of electromagnetic energy is related to its electrical dimensions, that is, to some characteristic length or diameter of the conductor compared to the wavelength of the energy being radiated. For relatively low frequencies on the order of 100 MH.sub.z or less and relatively low gain on the order of 3 dbi, or less, monopoles are practical antennas. Among the most common relatively low frequency monopole antennas are stubs, cones, cylinders, and whips.
A whip antenna is a monopole antenna having a length to radius ratio on the order of 1000. When such an antenna is sized to a quarter wavelength, it is capable of performance, at best, only over a relatively narrow frequency band width of about 1.1 to 1. This narrow bandwidth is not suitable for many antenna applications.
The bandwidth of a monopole antenna is increased by increasing its thickness or diameter. For example, an antenna with a length to radius ratio on the order of 50, commonly referred to as a stub or cylindrical antenna, may have a frequency bandwidth of about 1.35 to 1. Its minimum length can be reduced to about 0.15 times the wavelength. In the limit, cylindrical antennas can be constructed with a length to radius ratio on the order of 10. Such a cylindrical antenna may operate efficiently over a 3 to 1 frequency band width.
Conventional cylindrical antennas have certain inherent disadvantages which the present invention is designed to overcome. One of these disadvantages resides in the weight of the cylindrical antenna and its lack of adaptability to a deployable configuration. Thus, a conventional cylindrical antenna is essentially a relatively large diameter metal tube. As a consequence, the cylindrical antenna is relatively heavy and quite ill-suited to if not totally incapable of utilization in a deployable configuration. Another disadvantge of a conventional cylindrical antenna involves impedance matching of the antenna and its coaxial feed line. In order to provide an impedance match between the antenna and its coaxial feed line, the base end of the cylindrical antenna element or tube must be progressively reduced or tapered to the diameter of the inner coaxial conductor, which is difficult to accomplish. Without such a tapered impedance matching portion on the antenna, an impedance mismatch will exist which will significantly decrease the antenna gain.